1. Field of the Invention
The present invention relates to a fixing apparatus for fixing toner transferred onto a medium in electronic photograph ic recording equipment such as photocopiers, laser printers and facsimile machines and, more particularly, to a fixing apparatus for melting and fixing toner.
2. Description of the Related Art
For conventional electronic photograph recording equipment, there have been commonly available two types of apparatuses for fixing toner transferred onto a medium; one is a pressure fixing apparatus which pressurizes a medium while passing it between two rollers and the other is a thermal fixing apparatus which fixes toner while applying a pressure to the medium and melting it by heat from the rollers or the like.
In a pressure fixing apparatus which pressurizes the medium with pressure rollers, it is difficult to fix toner on a medium with irregularities, and moreover, a pressure fixing apparatus normally requires excessively large pressure to pressurize a medium, and therefore, a thin medium will be easily broken. Furthermore, to uniformly pressurize a medium in its widthwise direction, the diameter of the pressure rollers should be increased and the bearings which support the pressure rollers should also be large- sized and a large pressure roller driving force is duly required.
On the other hand, in some thermal fixing apparatuses, a heat source is incorporated in respective pressure rollers, or a medium is heated by a xenon lamp or the like without touching the medium. However, thermal fixing apparatuses will require a large capacity heat source and consume significantly large amounts power. Therefore, such thermal fixing apparatuses remain in danger of combustion at all times during use. Though these thermal fixing apparatuses are used in large number because they are relatively congenial to any kind of medium, for example, it takes a lot of time to thermally start up the apparatuses, so they cannot fix toner immediately. Moreover, thermal fixing apparatuses require large capacity heat source, and therefore, they are required to have provisions for discharging heat, fire prevention, and safety. As described above, there have been problems which prevent both pressure fixing and thermal fixing apparatuses from being reduced in size.
Under the circumstances, an ideal thermal fixing apparatus which requires less power consumption and is available for instantaneous fixing only by heating a least required part of the medium has been demanded. To satisfy such a demand, an apparatus using a thermal head as a heat source has been proposed. In this type of thermal fixing apparatus, a thermal head is arranged to oppose a medium onto which toner is transferred and the toner is melted by heat from this thermal head. A sheet which moves synchronized with the medium is arranged between the thermal head and the medium so as not to disturb toner before fixing.
In this type of thermal fixing apparatus, however, the sheet arranged between the thermal head and the medium prevents satisfactory heat conduction of heat from the thermal head to the medium. Thermal conductivity can be increased by reducing the thickness of the sheet, but it is technically difficult to thin the sheet because it conflicts with a need to maintain sufficient sheet strength. In short, there has been a problem that efficient conduction of heat from the thermal head to the medium is prevented by this sheet disposed therebetween, thus deteriorating the thermal efficiency.
In addition, a serious problem has arisen that the heating part of the thermal head wears because it is rubbed by a running sheet.